A 45-year-old man who is a heavy smoker complains of sudden onset of high fever and chills, nonproductive cough, and chest pain. An X-ray film of the chest shows bilateral, diffuse, patchy infiltrates. The patient reports that a number of similar cases have occurred recently in the building where he works. He responds to antibiotics and recovers. Which of the following is the most likely a reservoir for the microorganism that is responsible for this patient's respiratory tract infection?